A combination of pharmacological, molecular and physiological data suggest the presence of multiple subpopulations of nicotinic acetylcholine receptors (nAChR) in the central nervous system. To date, molecular biologists have identified eleven distinct nAChR subunits in neuronal tissue (five such subunits are believed to combine to form the functional nAChR), and over half a dozen different combinations of these subunits, when expressed together in various cell lines, produce functional, ligand-gated ion channels. However, only three distinct nAChRs can be identified using chemical ligands, and two of these are distinguished by polypeptide toxins. We have found differences between the structure-activity relationships (SARs) for various nicotine- mediated in vivo effects and in vitro affinity. We propose to continue these studies by synthesizing and testing a series of 5- and 6- substituted isonicotines, and to compare these with the SARs of previously reported 5- and 6-substituted nicotines. We additionally propose to synthesize some isonicotine analogs with an ether link between the 3-positions of the pyridine and pyrrolidine rings. We also propose to study the SAR of substituents attached at the 2'- and 4- positions of nicotine, as well as to the comparable positions on a series of pyrrolidine ring-opened analogs of nicotine. Susceptibility to mecamylamine antagonism is a hallmark of agonists to the central nicotine high affinity site, but this may not be the case for some of the 2'- and 4-substituted derivatives we are proposing. Additionally, we propose to synthesize analogs of trans-metanicotine, a central nicotine agonist with in vitro and in vivo selectivity, in which the carbon-carbon double bond off the 3-position of the pyridine ring is replaced by a bioisosteric sulfur atom. This modification will allow introduction of oxygen atoms (sulfoxide and/or sulfone) to mimic the structurally and pharmacologically similar pseudooxynicotine. We further propose to synthesize a series of N6-substituted 6- aminonicotines and 6-substituted nicotines as potential competitive nAChR antagonists. Key to these studies is a comprehensive approach to pharmacological evaluation of the various series of compounds. In vitro and in vivo evaluation will be needed for most of these compounds, as we have previously found instances where affinity and activity can vary independently of each other. We have a standard series of tests for evaluation of nicotine-like activity, including displacement of [3H]nicotine from rat brain, two functional assay using mice (inhibition of spontaneous activity and tail-flick antinociception) and drug discrimination using rats trained on nicotine. We additionally propose to utilize the newer in vitro expression systems to correlate selectivity with affinity for specific subunit combination using an oocyte expression systems (alpha4beta2, alpha3beta2).